Gas turbine drive



R. C. ALLEN Nov. 24, 1942.

GAS TURBINE DRIVE Filed Feb. 20, 1941 Patented Nov. 24, 1942 UNITEDSTATES PATENT OFFICE GAS TURBINE DRIVE Robertv C. Allen, Wauwatosa,Wis., assigner to Allis-Chalmers Manufacturing Company, Milwaukce, Wis.,a corporation of Delaware Application February 20, 1941, Serial No.379,774

(Cl. Gil-41g) llClaims.

This invention relates generally to prime movers driven by combustiongases or other noncondensable gases and more particularly to anarrangement including two or more of such prime movers or prime moversections operatively connected with a driven element so that when one ofsaid prime movers or prime mover sections is operating to drive saidelement, another one of said prime movers or prime mover sections isinoperative and running idle.

The propulsion of ships by means of continu- Gas turbines operate atrelatively high y quired to drive the idle turbine has reduced theoverall eiliciency to a degree suiiicient to render the use of gasturbine drives of this nature highly impractical.

Consequently, the primary object of this invention is to provide animproved drive embodying a novel correlation of elements including twoor more prime movers or prime mover sections driven by non-condensablegases and operatively connected with a driven element so that when oneof said prime movers or prime mover sections is operating to drive saidelement, another prime mover or prime mover section can run idle withoutmaterially decreasing overall emciency.

Another object of this invention is to provide an improved and novel gasturbine drive combination including two or more turbines or turbinesections, a sealing means and a vacuum producing means arranged so thatthe turbines or turbine sections always run idle in a high degree ofvacuum, thereby materially reducing the losses inherent in the known gasturbine drives.

The invention accordingly consists of the various features ofconstruction, combinations of elements and arrangements of parts as morefully pointed out in the appended claims and in the detaileddescription, in which:

Fig. 1 is a schematic illustration of a combustion gas turbine systemembodying the invention;

Fig. 2 is a partial sectional view illustrating the form of fluid sealedvalves employed in the system shown in Fig. 1:

Fig. 3 is a partial sectional View illustrating the general form oflabyrinth shaft seals employed; and

Fig. 4 diagrammatically illustrates the form and arrangement oi thecontrol cams shown in Fig. 1.

' Referring to Fig. l, it is seen that the illustrated marine drivearrangement comprises an ahead turbine I, an astern turbine 2, and apropeller shaft 3 which is continuously connected with the ahead turbineshaft 4 and with the astern turbine shaft 5 by means of reduction'gearing i. The .arrangement is such that when the ahead turbine'l isoperating t0 drive the propeller shaft 8, the astern turbine 2 isrunning idle at a commensurate speed.

Motive fluid for operating the ahead and astern turbines I and 2,respectively, is produced by a continuous combustion gas generating unitcomprising a compressor 1, a combustion chamlber 8 provided with a fluidfuel burner 9, and a gas turbine III. The compressor 1 and the turbineIII are preferably mounted on a common shaft II adapted for connectionwith an internal combustion starting engine I2 by means of a -clutch I3.Air, which enters the compressor 1 through an inlet conduit I3, iscompressed and discharged into the combustion chamber 8 through a pipeI4. 'I'he combustion gases produced in the combustion chamber 8discharge into a conduit I6 which in turn is connected with the inlet oradmission ends of the turbines I, 2 and I0 by means of pipes I1, I8 andI9, respectively. Turbines I and 2 exhaust into the pipes 2l and 22,respectively, which merge into a common exhaust conduit 23 connectedwith a heat exchanger 24 operatively associated with the air conduit I4connecting the compressor 1 with the combustion chamber 8. The turbineI0 also exhausts into the heat exchanger 24 through a pipe 28. Theexhaust gases entering the heat exchanger 23 are discharged toatmosphere through a conduit 21. Fluid fuel from a suitable source 28 is'delivered under pressure to the burner 9 by means of pipe 29, pump 3|and pipe 22. A portion of the pipe V32 passes through a. heat exchanger33 operatively associated with the conduit 21. 'I'he flow of fuel to theburner S is varied by a suitable regulating valve 38 disposed in thepipe 32.

The inlet and exhaust conduits of the ahead turbine I are provided withspring biased control valves 34 and 38, respectively. 'I'he inlet andexhaust conduits of the astern turbine 2 are also provided with spring!biased control valves 81 turbines I and 2.

and 38, respectively. The valves 34, 36, 31 and 38 are similarlyconstructed to embody a rpair of annular sealing grooves 39 formedbetween the valve casing 40 and the gate member 4| on the inlet anddischarge sides of the valve and adapted for connection with a suitablesource of sealing fluid, as best shown in Fig. 2. In this illustration,the sealing fluid, which consists of low pressure steam generated in aboiler 50, is conducted to the sealing grooves in the valves 34 and 36through pipe 42, pipe 43 and branch pipes 44 and 46, respectively.Sealing fluid is also conducted to the sealing grooves in the valves 31and 38 through pipe 42, pipe 41, pipe 48 and branch pipes 49 and 5|,respectively. The arrangement is such that when the valves 34, 36, 31and 38 are closed, the admission of sealing fluid to the pairs oflannular grooves 39 prevents the motive fluid and/or air from enteringthe interiors of the A portion of the steam generated in the boiler 50is used to drive a steam turbine 52 which in turn drives a generator 53for supplying power to the ships auxiliaries (not shown). The steam,which is conducted to the turbine 52 through pipes 42 and 54, exhauststhrough a conduit 51 into a condenser 58 having its hot well 59connected with the lower portion of boiler 50 by means of pipe 6|, pump62 and pipe 63.

The opposite ends of each of the turbines and 2 are provided withsimilar labyrinth shaft seals 64 and 66, respectively, which arepreferably constructed to permit the introduction of a suitable sealingfluid ata point intermediate the ends of the seals, as best shown inFig. 3. Low pressure steam from the boiler 50 is introduced into each ofthe seals 64 through the pipe 42, pipe 61 and branch pipes 68 and 69,and into each of the seals 66 through pipe 42, pipe 41, pipe 1| andbranch pipes 12 and 13. The flow of sealing fluid to the valves 34 and36 and the seals 64 and to the valves 31 and 38 and the seals 66 iscontrolled by the spring biased valves 14 and 16, respectively. Theinterior of each of the turbines I and 2 is operatively connected withthe interior of the condenser 58 by means of the common pipe 11 and thebranch pipes 18 and 19, respectively. Communication between theinteriors of the turbines I and 2 and the condenser 58 is controlled byspring biased valves 8| and 82 disposed in pipes 18 and 19,respectively.

The valves 34, 36, 31, 38, 14, 16, 8| and 82 are urged by theirrespective biasing springs toward their fully open positions and thestem portion of each valve is similarly provided with an identical camengaging roller 83. A cam shaft 84 is mounted for angular movement inbearings 86 and is provided with cams 81, 88, 89 and 8| arranged forcontinuous engagement with the rollers 83 carried by the stem portionsof valves 8|, 36, 34 and 14, respectively. Another cam shaft 92 ismounted for angular movements in bearings 93 and is provided with cams94, 96, 91 and 98 which are similar to cams 81, 88, 89 and 9|,respectively, and which are arranged for continuous engagement with therollers 83 carried by the stem portions of valves 82, 38, 31 and 16,respectively. The cam shafts 84 and 92 are also provided with handwheels 99 and |0| and with cams |02 and |03, respectively. Cams |02 and|03 are each provided with spaced projections |04 and |06, as shown inFig. 4 to limit the angular movements of their respective shafts.

The arrangement is such that when the turbines and 2 are both idle thecam shafts 84 and 92 are in the positions shown in Fig. 1, the rollers83 on the stem portions of valves 8|, 36, 34 and 14 and on the stemportions of valves 82, 38, 31 and 16 engage their respective cams in themanner shown in Fig. 4. That is, the rollers 83 on the stem portions ofvalves 8| and 82 engage the low portions of cams 81 and 94,respectively; the rollers 83 on the stem portions of valves 36 and 38engage the high portions of cams 88 and 96, respectively; the rollers 83on the stem portions of valves 34 and 31 engage the high portions ofcams 89 and 91, respectively; and the rollers 83 on the stem portions ofvalves 14 and 16 engage the low portions of cams 9| and 98,respectively.

The cams 94, 96, 91 and 98 on shaft 92 are identical in construction andarrangement to cams 81, 88, 89 and 9|, respectively, on shaft 84.Consequently, since the cams on the shaft 92 are also constructed andarranged as shown in Fig. 4, it should be obvious that when the shafts84 and 92 are positioned as shown in Fig. 1 the valves 8|, 82, 14 and 16are in their fully opened position. Moreover, it should also be obviousthat the inlet and exhaust conduits |1 and 2|, respectively, of turbineand |8 and 22, respectively, of turbine 2, are closed, that the sealingfluid conduits are open which permits sealing fluid to flow to thevalves 34 and 36 and the shaft seals 64 associated with turbine and tothe valves 31 and 38 and the shaft seals 66 associated with turbine 2,and that the conduits connecting the interiors of the turbines and 2with the interior of the condenser 58 are open which evacuates theturbines. Referring particularly to Fig. 4, it should also be noted thatthe configuration of the cams 81, 88, 89 and 9| is such that a movementof shaft 84 in a clockwise direction through an angle of approximatelyforty degrees from the position shown (stop |04 on cam |02 prevents amovement of the shaft in a counterclockwisedirection) effects a rapidclosure of valves 8| and 14, a rapid opening of the exhaust valve 36,and a gradual partial opening of the admission valve 34 and that if thismovement of the shaft is continued until further movement is preventedby the stop |06 on cam |02, during such movement the valves 8| and 14remain fully closed, valve exhaust 36 remains fully open, and admissionvalve 34 is gradually moved to its fully open position. With'the partsin this position, a movement of the shaft 84 in a counterclockwisedirection to the position shown in Fig. 4 rst effects a gradual closingof admission valve 34 and when the admission valve 34 actually closes, asudden closure of exhaust valve 36 and a sudden opening of valves 14 and8|. The valves 82, 38, 31 and 16 associated with the turbine 2 aresequentially actuated by means of the shaft 92 and cams 94, 96, 91 and98, respectively, in the identical manner just described with respect tothe valves 8|, 36, 34 and 14 associated with the turbine The quantityand the energy content of the combustion gases produced in thecombustion chamber 8 for operating the turbine |0 and either turbine or2 is varied in accordance with load requirements by regulating the fuelsupply to the burner 9 and the speed of the turbine |0 and thecompressor 1. This regulation is accomplished in the illustratedarrangement by a speed responsive governor including a flyball mechanism|01 operatively connected with the shaft which is common to the turbine|0 and the compressor 1, by means of the shaft |08 and the gearing |09.The tlyball mechanism |01 includes a pair of weights ||0 and upper andlower sleeve members and H2, respectively, which are slidably mounted onthe upper portion of shaft |08 and which are operatively connected withthe weights ||0. The upper sleeve member when in its lowermost positionengages the adjacent end of the member I6 formed on or secured to theshaft |08, and is provided at its upper end with a collar portion I1operatively connected with one end of a lever ||8 which has its otherend pivotally connected with a xed fulcrum H9. The lower sleeve member||2 is also operatively connected with one end of a lever |2| having itsother end pivotally connected with one end of a lever |22 by means of alink |23. An intermediate portion of lever |2| is pivotally connectedwith an intermediate portion of lever ||8 by means of a link |24 andanother intermediate portion of lever |2| is operatively connected withthe stem portion of the fuel regulating valve 30 by means of 'a link |26and the bell crank lever |21. A compression spring |28 is disposed inabutting relation between the member I6 on the shaft |08 and the upperend of the lower sleeve ing spring |32 operative to maintain the roller|3| on the end thereof in continuous engagement with the cam |02 onshaft 84. An intermediate portion of lever |22 is pivotally connectedwith one end of a link |33 which carries at its other end a cam engagingroller |34 and which has associated therewith a biasing spring |36operative to maintain the roller |34 on the end thereof in continuousengagement with the cam |03 on the shaft 92.

With the parts in the various positions shown, the turbines and 2 areboth idle, since their respective admission and exhaust valves 34, 36and 31, 38 are all closed and the combustion gas generating unitcomprising the compressor 1, the combustion chamber 8 and the turbinel0, assuming operation of the unit has been initiated in the usualmanner by employing vthe internal combustion starting engine |2, isoperating under no load or idling conditions and the fuel regulatingvalve 30 is in its minimum closed position and the speed governor is setto main-` tain the speed of said unit substantially constant at apredetermined minimum. When it is desired to propel the vessel in theahead direction,

all that has to be done is to move the hand wheel 99 secured to camshaft 84 in a clockwise direction until the admission valve 34 opens.movement of hand wheel 99 effects a complete closure of valves 14 and 8|and a complete opening of exhaust valve 36 before admission valve 34actually begins to open and in addition permits the biasing spring |32associated with the link |29 to'eiect a gradual upward.` movement of thelink (note that the configuration of the cam |02 is similar to cam 89 asshown in Fig. 4), which in turn causes lever |22 to move in acounterclockwise direction about its point of pivotal connection withlink |33. This movement of lever |22, which preferably slightly precedesthe actual opening of admission valve 34, effects a downward movement oflink |23 and a clockwise movement of lever |2| about its point ofpivotal connection with link |24, which in turn moves the link |26 andbell crank lever |21 in This a direction to eifect an opening movementof the 75 fuel valve 30. This movement of lever 2| also moves the lowersleeve member ||2 upward which compresses the spring |28. However, sincethe upper and lower sleeve members and |2 are interconnected through theflyballs ||0, the upward movement of the lower sleeve member ||2 alsoeffects a corresponding upward movement of the upper sleeve member andas a result lever IIB pivots about its xed fulcrum ||9 in a clockwisedirection and by means of the link |24 eiects a corresponding pivotalmovement of lever |2| about its point of pivotal connection with link|23. This results in the link |28 being moved upward, which in turneiects a counterclockwise movement of bell crank lever |21 and a closingmovement of the fuel regulating valve 30. In this connection, the leverand link arrangement is such that the opening movement of valve 30effected bythe pivotal movement of lever |2| about its point of pivotalconnection with link |24 is greater than the closing movement thereofeffected by the interconnection between the levers |2| and ||8 and theresultant pivotal movement of lever |2| about its point of pivotalconnection with link |23. In other words, a suicient movement of thehand wheel 99, which is secured to shaft 84, in a direction to effect anopening movement of the admission valve 34 simultaneously moves thevalve 30 towards its fully opened position and increases the fuel supplyto the burner 9 which in turn increases the energy content of thecombustion gases and thereby the speed of the turbine |0 andcompressor 1. The net result is that the quantity and the energy contentof the coinbustion gases are simultaneously increased as the valve 34,which controls the admission of motive uid to the turbine is movedtoward its fully open position. Conversely, when the turbine isoperating and the admission valve 34 is moved toward its fully closedposition, the quantity and the energy content of the combustion gasesare correspondingly decreased by moving the valve 30 towards its closedposition which in turn decreases the energy content of the combustiongas and thereby the speed of the turbine |0 and compressor 1.

When the combustion gas turbine unit comprising the compressor 1, thecombustion chamber 8. which includes the burner 9, and the. turbine I isoperating (operation of this unit is terminated either by closing a fuelshut-off valve, not shown, or by stopping the pump 3|), the position ofthe fuel regulating valve 30 can be varied either by moving cam |02 or|03 on the control shafts 84 and 92, respectively, or by the governorflyball mechanism |01 in response to variations in the speed of theturbine |0. Variations in the speed of the turbine |0 may occur due toappreciable changes in the temperature of the air which is beingcompressed and to unequal distribution of the gases between the turbineI0 and the power turbines or 2. When a change in speed does occur, forexample, a decrease in speed, the spring |28 causes the collar ||2 tomove downward, thereby effecting a counterclockwise movement of lever|2| about its point of pivotal connection with link |23, which in turnmoves the link |26 and bell crank lever |21 in a direction to more fullyopen the valve 30. The downward movement of collar ||2 effects acorresponding movement of collar and connected lever ||8 and link |24.

Obviously, as the speed increases the sleeve ||2 and the mechanismoperatively connected therewith are moved in opposite directions fromthat just described to effect a closing movement of the valve 30.

When the turbine I is operating in the manner just described, the camshaft 92 and the associated valves 82, 38, 31 and 1S are in thepositions shown; i. e., the valves 82 and 16 are fully open and thevalves 38 and 31 are fully closed. Consequently, the inlet and exhaustconduits I8 and 22, respectively, of the turbine 2 are closed, thesealing fluid (low pressure steam) flows from the boiler U into thesealing grooves 39 in the valves 31 and 38, the interior of the turbine2 is connected with the interior of the condenser 58, and the rotor ofthe turbine 2 is running in a vacuum which materially decreases thelosses normally experienced with this type of prime mover.

When the vessel is being propelled in the ahead direction, by. theturbine I and it is desired to propel the vessel in the asterndirection, the hand Wheel 99 on shaft 84 is moved in a direction toeiect a closure of admission valve 34 and, as soon as the admissionvalve actually closes, the exhaust valve 36 closes and the valves 14 and8l are fully opened. The opening of valve 14 admits sealing uid to thesealing grooves in the valves 34 and 36 (note the construction shown inFig. 2) and into the shaft seals 64 (see Fig. 3). The opening of valve8l connects the interior of the turbine with the interior of thecondenser 58 and since both the inlet and exhaust conduits are fullyclosed by the valves 34 and 36, respectively, the rotor of the turbineis running in a vacuum; i. e., the interior of the turbine is beingevacuated by the condenser 58. All that now remains to be done to effectan operation of the turbine 2 is to move 4 the hand wheel IUI, securedto cam shaft 92, in a direction to eiect an opening movement of theinlet valve 31 associated with turbine 2.

The movement of hand wheel IUI actuates the valves 16, 31, 38 and 82,which are associated with turbine 2, varies the setting of the governor,and varies the position of the fuel regulating valve 30 in exactly thesame manner as described in connection with the movements of the handwheel 99. Consequently, a repetition of the operation in this respectshould not be necessary for a complete understanding of the invention.However, it should be particularly noted in this connection thatwhenever the flow of motive uid to either turbine I or turbine2 isterminated by the closure of its inlet valve, its exhaust valve is alsoclosed, a condensable sealing fluid is admitted to the sealing groovesin its inlet and exhaust valves and into its shaft seals, and theinterior of the turbine is connected with the interior of a condenser.

The invention is applicable generally to prime moverarrangements inwhich the prime movers are driven by non-condensable gases and in whichtwo or more of such prime movers or prime mover sections are operativelyconnected with a driven element so that when one of said prime movers orprime mover sections is operating to drive said element, another of saidprime mover or prime mover sections is inoperative and running idle andit' should be understood that although the invention has beenillustrated in connection with a continuous combustion gas turbinemarine drive arrangement, it it not intended to limit the invention tothe exact mode of operation and to the exact details of constructionherein shown and described, as various modifications within the scope ofthe appended claims may occur t'o persons skilled in the art.

It is claimed and desired to secure by Letters Patent:

1. In combination, a power plant including a power developing rotorelement enclosed in a casing, a source of non-condensable gas foroperating said rotor element, an evacuating means connected with theinterior of said casing, means controlling the flow of non-condensablegas to said rotor element, means controlling the connection between saidevacuating means and the interior of said casing, and control mechanismoperatively associated with said non-condensable gas ow control meansand with said connection control means so as to connect said evacuatingmeans with the interior of said casing whenever the flow ofnon-condensable gas to the rotor element in said casing is terminated.

2. In combination, a power plant including two encased power developingrotor elements operatively connected together for simultaneous rotation,a source of non-condensable gas for operating said rotor elements, anevacuating means connected with the interior of the casing of each rotorelement, means for selectively controlling the flow of non-condensablegas to said rotor elements, means selectively controlling the connectionbetween said evacuating means and the interior of each of said casings,and control mechanism operatively associated with said noncondensablegas flow control means and with said connection control means so as toconnect said evacuating means with the interior of the casing of theoperating one of said elements whenever the flow of non-condensable gasto the said one of the rotor elements is terminated.

3. In combination, a power plant including a power developing rotorelement enclosed in a casing, a source of non-condensable gas foroperating said rotor element, a first means controlling the flow ofnon-condensable gas to said rotor element, meanson said element andcasing coacting to provide labyrinth seals adjacent the ends thereof, anevacuating means, a second means for connecting said evacuating meanswith the interior of said casing, a source of sealing fluid, a thirdmeans for connecting said labyrinth seals with said source, and controlmechanism operatively associated with said rst, second and third meansto terminate the flow of non-condensable gas to said rotor element, toconnect the interior of its casing with said evacuating means, and toconnect said seals with the said source of sealing fluid.

4. In combination, a power plant including a power developing rotorelement enclosed in a casing having gas inlet and exhaust connections, asource of non-condensable gas for operating said element, a rst meansoperative to control the ow of non-condensable gas through said inletand exhaust connections, an evacuating means connected with the interiorof said casing, a second means controlling the connection between saidevacuating means and the interior of said casing, and control mechanismoperatively associated with said first and second means so as to connectsaid evacuating means with the interior of said casing Whenever the flowof non-condensable gas through said inlet and exhaust connections isterminated.

5. In combination, a power plant including a power developing rotorelement enclosed in a casing having gas inlet and exhaust connections, asource of non-condensable gas for operating said element, means on saidelement and casing coacting to provide labyrinth seals adjacent the endsthereof, a first means operative to control the flow of non-condensablegas through said inlet and exhaust connections, an evacuating means,means operative to connect said evacuating means with the interiorofsaid casing, a source of sealing fluid, a second means operative toconnect said labyrinth seals with said source of sealing fluid, acontrol mechanism operatively associated with said first and secondmeans and with said evacuating connecting means to terminate the ow ofnon-condensable gas through said inlet and exhaust connections, toconnect said labyrinth seals with said source of sealing fluid, and toconnect said evacuating means with the interior of said casing.

6. In combination, a power plant including a power developing rotorelement enclosed in a casing having gas inlet and exhaust connections, asource of non-condensable gas for operating said element, means on saidelement and casing coacting to provide labyrinth seals adjacent the endsthereof, a iirst means operative to control the ow of gases through saidinlet and exhaust connections, a source of condensable gas, a condenseroperatively connected with said source of condensable gas, a secondmeans operative to connect said labyrinth seals with said source o!condensable gas, a third means operative to connect said condenser withthe interior of said cuir-ing, and control mechanism operativelyassociated with said first, second and third means to terminate the owof'non-condensable gas through said inlet and exhaust connections, toconnect said labyrinth seals with the said source of condensable gas,and to connect said condenser with the interior of said casing.

y7. In combination, a power plant including a combustion gas turbine, afirst means controlling the iiow of combustion gases to said turbine, anevacuating means connected with the interior of said turbine, a secondmeans controlling the connection between said evacuating means and theinterior of said turbine, and control mechanism operatively associatedwith said rst and second means so as to connect said evacuating meanswith the interior of said turbine casing whenever the ilow of combustiongases to said turbine is terminated.

8. In combination, a combustion gas turbine system including ahead andreverse turbine elements operatively connected together, a rst means forselectively controlling the flow of combustion gases to said turbineelements, an evacuating means connected with the interior of eachturbine element, a second means for selectively controlling theconnection between said evacuating means and the interior of eachturbine element, and control mechanism operatively associated with saidilrst and second means so as to connect said evacuating means with theinterior of the operating one of said turbine elements whenever the owof combustion gases through the said one turbine element is terminated.

9. In combination, a combustion gas turbine system including ahead andreverse turbine elements operatively connected together and capable ofdeveloping power independently of each other, said turbine elements eachincluding labyrinth seals operative to prevent motive iluid fromescaping along the shaft portion of the turbine element, a source ofcondensable gas, a first means operative to connect said labyrinth sealswith the said source of condensable gas, a condenser, a second meansoperative to connect the interior of the casings enclosing said turbineelements with said condenser, and control mechanism operativelyassociated with said first and second means to terminate the ilow otmotive fluid to the operating one of said turbine elements and toconnect the interior of its casing with `said condenser and itslabyrinth seals with the said source oi.' condensable gas.

10. In combination. a combustion gas turbine system including ahead andreverse turbine elements enclosed in casings each having inlet andexhaust connections, means on each of said elements and its associatedcasing providing labyrinth seals adjacent the ends thereof, a rst meansoperative to control said inlet and exhaust connections, a source ofcondensable gas, a second means operative to connect said labyrinthseals with said source of condensable gas, a condenser, a third meansoperative to connect said condenser with the interior oi said casings,and control mechanism operatively associated with said iirst, second andthird means so as to close the inlet and exhaust connections to one ofsaid casings and so as to then connect its labyrinth seals with the saidsource of condensable gas and the interior of its casing with the saidcondenser.

1l. In combination, two encased prime mover rotor'elements, a source ofnon-condensable gas for operating said elements, a first means forselectively connecting said source with said elements to effect anindependent operation thereof, a device to be driven by said elements,means operatively connecting said elements with said device so that whenone of said elements is driving saiddevice the other of said elements isalso rotatingat a commensurate speed, an'evacuating means connected withthe interior of the casing of each element, a second means forselectively controlling the connection between said evacuating means andthe interior of each of said casings, and a control operativelyassociated with said ilrst and second means so as to connect saidevacuating means with the interior of the casing 'of one of saidelements whenever the ilow of gas to the said one of said elements isterminated.

l ROBERT C. ALLEN.

